Friday, January 25, 2013

The winter storm system moving across the Carolinas today is bringing all three types of precipitation -- snow, sleet and freezing rain.

We won't have to worry much about snow, as a "nose" of warm air several thousand feet up in the atmosphere is melting the precipitation as it falls. Some of that is refreezing into sleet, and some is reaching the ground as rain -- then freezing.

Anyways, since we're seeing a little of everything this morning, here's a quick primer on how it forms. And I'm including my two cents' worth on what's worse ...

SNOW

This requires cold air in all levels of the atmosphere. The only areas seeing snow today are in the northwest mountains, because the "warm nose" (I'll explain this later) isn't expected to reach as far north as the Boone area.

Power outage impact: Usually none, unless the snow is very heavy.

Road impact: Considerable, although crews usually can manage snow with plows and salt.

SLEET

For sleet to fall, we need a relatively thin column of warm air in the middle levels of the atmosphere. Precipitation starts falling as snow, melts on its way down and forms rain droplets, then freezes back into ice pellets as it moves within a few thousand feet of the surface.

Power outage impact: None.

Road impact: Intense. Sleet causes big problems on the roads, as it tends to melt when reaching the surface and then freezes immediately. Also, sleet tends to fall when precipitation is heavier. The overwhelming volume of sleet causes pockets of frozen pebbles to form on roads.

FREEZING RAIN

For freezing rain to fall, we need cold air at the upper levels, and then relatively warmer air all the way to the ground. But the surface levels have to be at or below freezing.

Power outage impact: If ice accumulates 1/4 of an inch, tree limbs can fall onto power lines, and we have trouble. When ice accumulates to 1/2 inch or more, there can be widespread problems. Accumulations in the Charlotte area from today's system are only predicted to be 1/10 of an inch.

Road impact: Often, the main travel lanes on major roads remain in pretty good shape during freezing rain. Ice tends to form on the curbs -- and, of course, on bridges and overpasses, where the road surface is colder. Sidewalks are especially dangerous. One other thing -- downhill sections of roads tend to get ice, too. I'm sure there some explanation for this, but you'd have to ask a Physics teacher.

9
comments:

Yes We Can..and Will Do More
said...

Thank you for these definitions! Very helpful. All of these come from global warming according to Al Gore, who has selflessly dedicated his life to making us aware. He should have been president in 2000.

Remember the liberal handbook requires the use of Global Warming in warm months and Climate Change in winter. Remember, Professor Curry, a former US National Research Council Climate Research Committee member and the author of more than 190 peer-reviewed papers said we should, "communicate honestly with the public about what we know and what we don’t know about climate change. Take a lesson from other scientists who acknowledge the “pause”.’

In addition, the Met Office and the Climatic Research Unit (CRU) at East Anglia University now confirms on its climate blog that no significant warming has occurred recently, (the past 16 years).

So if we use the term climate change when cold, and global worming when hot, we can have a greater chance to enact some of Al Gore’s ideas, like carbon credits, which will help to raise utility rates, and limit the U.S ability to use its own natural resources, while helping a group Al Gore feels greatly matches his own views. The oil rich country of Qatar's government owned and controlled Al Jazeera.

Al Gore has done a lot of legwork in getting the news out. Perhaps no one, not even Obama, has done more to advance progress and inform the public of what is best for them in the past decade than Gore.

In an age of few great men, Al Gore should be embraced as a visionary, one who can make a difference, and make us all aware of what should really matter.

You know, I think it is sad, and a little pathetic, that a simple little article about winter weather - which had NOTHING political in or about it - needed to be seen as an excuse by some people to make political commentary about climate change.

Poor Steve must be thinking, "Gee. I was just trying to be helpful and a little bit funny on a bad weather day. Now, I wish I'd never written the damn thing."

The fate of our future is more urgent that Steve's forecast, even though he does a good job. The most important thing is that we are aware of what will soon happen to our planet due to global warming. Sleet and snow is small beans compared to the loss of millions of lives in the coming decades when the sea rises. We only need to feel a sense of urgency about it, that is all.

How can it melt and then immediately refreeze? One possible explanation is that the melting point is dependent on temperature AND pressure.

For instance, when you are ice skating, the ice immediately underneath your skate blades are under high pressure, and the ice melts. Once you skate past, it refreezes. This is how you can slide easily with ice skates and you can't when you go on the rink wearing shoes.

What hapens to sleet is: at the moment of impact with the ground, the particle of sleet gets squeezed together. Thus it is under high pressure and it melts. Once the impact energy is dissipated, the water refreezes.

Now, some sleet particles "bounce". When they bounce, they don't refreeze. The term for when something bounces is called an "elastic collision"; when something hits and sticks, it is an "inelastic collision". Elastic collisions are "conservative" - all the kinetic energy remains in the particle and it bounces back with the same velocity that it had when it first hit, but in the opposite direction. So the particle is not forced to "squeeze" itself as it would if it hit and stuck - so the internal pressure doesn't get high enough to cause it to spontaneously melt.

In the real world, there are no "true" elastic collisions, they are all "partially inelastic". The difference between a sleet particle that melts and one that does not is how "inelastic" that particular collision turned out to be, and whether it raised the internal pressure enough to cause it to melt. This is dependent on many factors, so it's difficult to predict whether a particular storm will produce sleet that turns into ice on impact or sleet that bounces and doesn't. In fact, both can be happening - some sleet can bounce while others melt.

About this blog

Steve Lyttle says growing up in northeastern Ohio, with its foot-deep snows and summertime severe thunderstorms, fueled his interest in meteorology. He has written about weather for 10 years at The Observer. Join the discussion about weather trends and weird weather events -- whether local or around the world.